This invention relates to the field of non-destructive testing. In particular, the invention provides a method and apparatus which scans a test object with a laser beam to detect defects in the object.
Laser beams have been used, in the prior art, to test objects for defects. One example of non-destructive testing by laser techniques is shown in U.S. Pat. No. 4,408,881. The cited patent shows a system which creates a time-averaged hologram of the object, while the object is made to vibrate at a plurality of frequencies which are uniformly distributed over a preset range. The holographic image of the test object contains interference fringes in areas where there is a defect.
Another example of the use of coherent light for non-destructive testing is given in U.S. Pat. No. 3,604,253. The apparatus disclosed in the latter patent directs a laser beam through a lens, which projects the beam onto the entire object. The object is made to vibrate, and the pattern of light reflected from the object is analyzed. As the frequency and intensity of vibration are varied, changes appear in the pattern, indicating defects in the object.
U.S. Pat. No. 3,900,265 shows another system for detection of flaws in an object. The latter patent shows a device which scans a test object with a laser beam. The intensity of the reflected laser light is analyzed to produce an indication of flaws in the object. The patent discloses improved circuitry for accomplishing the scanning, and for prevention of errors in interpreting the results of the test.
Still another example of a device which scans an object with a laser beam is shown in U.S. Pat. No. 4,508,450. The patent discloses a directed laser which scans the surface of a flat object in a spiral pattern. The reflected light is received by a photodetector, and is compared with a reference signal to give information about defects in the object. The results of the test are displayed on a video monitor.
In U.S. Pat. No. 4,465,371, the distribution pattern of laser light reflected from a test object is examined. Comparison of this pattern with the known pattern of an unflawed object shows whether the object is defective.
The present invention provides another type of system, which is especially useful in detecting flaws in bonded structures, such as laminated materials and composites. The invention has the sensitivity of holography, but does not employ a holographic technique. Instead, the system examines the speckle pattern which results from illuminating a point on the object with coherent light. If there is a defect at a particular point, the speckle pattern obtained from that point will become blurred when the object is made to vibrate. The system of the present invention analyzes the speckle pattern obtained from each point on the object, and thereby determines the location of any defects. A more detailed discussion of the theory of operation of the invention will be presented below.
The apparatus of the present invention produces an image of the object without using holography. In particular, the invention does not employ a reference beam and an object beam, as is required in holography. Moreover, the invention records its results electronically, and not with a photographic process. Thus, the invention eliminates the need for the chemicals used in photographic development. Instead, the results of the analysis are stored in a computer for further review, and can be displayed graphically, on a video monitor. The system is programmed to display a false-color image of the object, which clearly shows the location of defects.
The apparatus also enjoys other important advantages. Unlike a holographic apparatus, it does not require a vibration isolation system. It also does not require any prior knowledge of the surface of the test object; there is no need for a detailed "reference" or "signature" signal, as is needed in some of the above-cited prior art. The invention also eliminates the need for many optical components such as lenses, beam splitters, and the like. Furthermore, because the apparatus scans only one point on the object at a time, it operates with a laser or relatively low power output.